Expression of structurally diverse Qa-2-encoded molecules on the surface of cloned cytotoxic T lymphocytes

Unusual expression pattern of the MHC class I Q5k gene in the AKR mouse: possible role in embryo development

Analysis of the tissue-specific expression of the Q5k gene in the AKR mouse reveals an unusual expression pattern. The Q5k mRNA is present in embryos from day 12, but expression is switched off in most tissues except thymus and testis shortly after birth. Late in pregnancy the gene is again transcribed in females. Analysis at the epitope level, with a Qa-2 specific monoclonal antibody revealed that in most cases the Q5k product is confined to the cytoplasm. These results suggest that Q5k has a most unusual tissue distribution and timing of expression among all the H-2 class I and Q genes so far described.

Extensive structural homology between H-2 K/D/L antigens and non-polymorphic class I Qa, Tla and "37" molecules suggests they may act as peptide carriers

Key structural features of H-2 K/D/L and HLA A/B/C class I molecules were identified by analysing the available sequences with reference to the 3-D structure of HLA-A2. Most of them were found to be conserved in a panel of 6 Qa and 4 Tla sequences. This finding, in addition to the high overall sequence similarity between polymorphic and non-polymorphic class I molecules strongly suggests a possible role for the latter in peptide binding, transport and/or presentation.

Molecular mapping of signals in the Qa-2 antigen required for attachment of the phosphatidylinositol membrane anchor

Proteins anchored in the membrane by covalent linkage to phosphatidylinositol (PtdIns) can be released by treatment with purified PtdIns-specific phospholipase C (Ptd-Ins-PLC). A recent survey of leukocyte antigens using flow cytometry has shown that staining of certain Qa antigens was diminished after PtdIns-PLC treatment, but staining of structurally related H-2 antigens was not affected. Therefore, in this study, the sensitivity of cell-surface Qa-2, H-2Kb, and H-2Db to hydrolysis by PtdIns-PLC was investigated biochemically by immunoprecipitation of radioiodinated molecules from cell lysates or supernatants. Qa-2, but not H-2Kb, was released from the surface of PtdIns-PLC-treated C57BL/10 mouse spleen cells and recovered in the cell supernatants. Similar analysis of thymoma cells transfected with cloned C57BL/10 genes showed that cell-surface Qa-2 molecules encoded by a Q7b cDNA and the Q7b or Q9b gene were sensitive to hydrolysis by PtdIns-PLC, whereas the H-2Kb and H-2Db gene products were resistant. Using thymoma cells transfected with hybrid genes constructed by exchanging exons between Q7b and H-2Db, the signals for PtdIns modification were localized to a defined region of Qa-2. This region differs from H-2Db most significantly by the presence of a central aspartate residue in the transmembrane segment and in the length of the cytoplasmic portion.

Tissue-specific expression of cell-surface Qa-2 antigen from a transfected Q7b gene of C57BL/10 mice

We screened a cDNA library prepared from a BALB.B10 CTL clone that expresses Qa-2 antigen, and isolated four clones derived from Q7b, a Qa region gene of C57BL/10. One of these Q7b cDNAs and the Q7b chromosomal gene were subcloned into expression vectors and transfected into L cells and R1.1 thymoma cells. We found that the chromosomal Q7b gene expresses Qa-2 on the surface of R1.1 cells, but not on L cells while the Q7b cDNA expresses protein on the surface of both cell types. The levels of Qa-2 expression do not correlate with the total levels of Q7b mRNA in these transfectants. Our results suggest that the tissue-specific expression of Qa-2 may be controlled, in part, by mechanisms of alternate RNA splicing. By using hybrid gene constructs, we have mapped the tissue-specific element to the 3' part of the gene, downstream of a site near the middle of exon 4. The hybrid polypeptides differ significantly in their transmembrane and cytoplasmic regions. These portions of the protein also may play a role in the tissue-specific expression of Qa-2.

Molecular complexity of Qa-2 antigens demonstrated by two-dimensional gel electrophoresis

Biosynthetically labelled Qa-2 antigens were isolated from mouse spleen cells by immunoprecipitation with anti-Qa-2 antisera. When newly synthesized Qa-2 molecules from several different inbred strains were analysed by two-dimensional (2D) gel electrophoresis; four different phenotypes were observed that differed in the number of polypeptides present. The ability to distinguish Qa-2+ phenotypes was used to map the recombination points in two congenic strains, B6.Tlaa and A.Tlab. No alternative Qa-2-like polypeptides were detected in B6.K1 (Qa-2-) cells using a polyspecific rabbit antiserum against mouse class I antigens, but a new molecule was detected in BALB/cBy (Qa-2-) cells. Pulse-chase and surface-labelling experiments showed that some, but not all, of the newly synthesized Qa-2 precursor forms are processed to mature cell surface molecules.

Qa alloantigen expression on functional T lymphocytes from spleen and thymus

The cell-surface expression of the class I alloantigen Qa-2 was analyzed on resting and activated spleen and thymus cells using cytotoxic elimination and immunofluorescence and flow cytometry. Spleen cells activated by mitogens or alloantigen were homogeneously positive for cell surface Qa-2, but activated splenic T cells expressed only about one-third as much Qa-2 per cell as did nonstimulated T cells. These data correlated with the ability to perform cytotoxic elimination with Qa-2-specific monoclonal antibodies (mAbs) in that cytotoxic T lymphocyte (CTL) activity was completely abrogated by pretreatment of spleen cells prior to in vitro culture but was only partially eliminated by treatment of CTL effectors. Qa-2-positive cells constituted only a small subpopulation of fresh normal thymocytes, but were enriched (greater than 40% positive) among cortisone-resistant thymocytes (CRT). These Qa-2-positive CRT contained mature thymocytes as defined by Ly phenotype Ly-2-, Ly-1hi. When normal thymocytes were treated with Qa-2-specific mAb and complement prior to in vitro sensitization for generation of allogeneic CTL, CTL activity was completely abrogated despite the fact that the fraction of cells eliminated were undetectable as assessed by cell recovery. CTL effectors from alloantigen-stimulated thymocytes were also susceptible to cytotoxic elimination with Qa-2-specific mAb. These data suggest that the Qa-2 molecule may serve not only as a marker on resting and activated peripheral T cells, but also as a unique marker for functionally mature T cells in the thymus.

Molecular biology of the H-2 histocompatibility complex

The H-2 histocompatibility complex of the mouse is a multigene family, some members of which are essential for the immune response to foreign antigens. The structure and organization of these genes have been established by molecular cloning, and their regulation and function is being defined by expression of the cloned genes.